The potential functional significance of a newly discovered form of synaptic plasticity observed in the mammalian hippocampal formation will be examined specifically with regard to its possible direct or indirect role in learning and memory operations. This phenomenon, which we call short-term exploratory modulation (STEM), results from behaviors that can roughly be categorized as exploratory in nature, but is independent of the motor components of this behavior. The primary aim of the proposal is to determine whether the STEM mechanism is one that could plausibly be used for high capacity information storage, or whether it represents a form of global system modulation, perhaps important for the storage process, but not directly involved in it. The proposed approach to these questions include: a determination of the extent to which STEM is related to the relatively-well-studied but artificial form of synaptic enhancement (LTE/LTP) that can be induced by convergent high- frequency activity on hippocampal excitatory pathways; an assessment of the role of known heterosynaptic modulation systems in the hippocampus; the correlation of STEM with actual learning; and an investigation of the information content of the STEM phenomenon. Specifically, five different lines of experimental questions will be addressed: 1) Is STEM found only in the temporal lobe afferents to the fascia dentata, or do other extrinsic or intrinsic hippocampal synapses show this form of spontaneous plasticity? 2) Do STEM and LTE occlude, is STEM regulated by NMDA receptor dependent mechanism, and does STEM require spontaneous activity on the perforant path fibers on which it is expressed? 3) Do heterosynaptic transmitter systems cause STEM or contribute permissively in some way? 4) Is STEM related to explicit or incidental spatial learning? Are STEM and learning ability positively correlated? 5) Does STEM reflect information-specific changes in different synaptic subsets? It is expected that these studies will further our understanding of the types of plasticity that occur in the mammalian hippocampus, their relation to learning and memory operations, and their potential contribution to normal cognition.